Novel catalytic performance of SnO2 for the steam reforming of methanol

Article abstract of DOI:10.1246/cl.2002.1078

Steam reforming of methanol took place over SnO₂ catalyst, not forming carbon monoxide appreciably. The reaction was suggested to proceed through the steam reforming of formaldehyde formed by the dehydrogenation of methanol.

Full text of DOI:10.1246/cl.2002.1078

1078
Chemistry Letters 2002
Novel Catalytic Performance of SnO2 for the Steam Reforming of Methanol
Arthit Neramittagapong, Shun-ichi Hoshino, Tohru Mori, Jun Kubo, and Yutaka Morikawaꢀ
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
(Received July 24, 2002; CL-020610)
Steam reforming of methanol took place over SnO2 catalyst,
not forming carbon monoxide appreciably. The reaction was
suggested to proceed through the steam reforming of formalde-
hyde formed by the dehydrogenation of methanol.
Methanol is often considered as a promising hydrogen source
for a fuel cell and an alternative energy source for automotives.1{3
Hydrogen can be produced from methanol through two different
processes, namely, steam reforming (CH3OH þ H2O !
CO2 þ 3H2) and partial oxidation (CH3OH þ 1/2O2 !
CO2 þ 2H2).4{6 For the purpose of supply to a fuel cell, the
minimization of CO formation occurring in both processes as a
side reaction is required, because CO acts as a poison to the
electrode of fuel cell.
Steam reforming of methanol has been performed on group
VIII metals4;5 or Cu-containing7{9 catalysts. The activity of Cu/
ZnO catalyst has investigated extensively in relation to the
dispersion of Cu particle8 or Cu/Zn ratios9 and the high
Figure 1. Effect of temperature on the steam reforming of
methanol over SnO2. Catalyst, 0.97 g; total flow rate,
37.5 cm3 minꢁ1; N2/CH3OH/H2O, 2/1/1.
6
10
performance of Cu/ZnO/ZrO2/Al2O3 and Cu/CeO2 catalysts
has been also reported. The problems on these catalysts are their
activities for methanol decomposition and water-gas shift
reaction. CO formed by the methanol decomposition reacts with
water present in the reaction system but remains in some extent
because of the equilibrium of water-gas shift reaction. CO
formation by the reverse water-gas shift reaction of steam
reforming products is also a matter for consideration. We reported
novel activity of SnO2 for the methanol conversion (eq 1)11 which
formed equal amounts of CO2 and CH4 and very recently have
found that the catalyst promotes the methanol steam reforming
unaccompanied by the water-gas shift reaction.
of the reaction temperature. CH4 was formed at ca. 10%
selectivity. As reported previously, SnO2 catalyzes selectively
the methanol conversion into CO2, CH4, and hydrogen. The
present result indicates that the methanol conversion was
suppressed by the presence of water vapor. Small amounts of
HCHO, CO, and dimethyl ether (not shown) were also produced.
The selectivity of HCHO decreased slightly with increasing
reaction temperature. It is to be noted that the selectivity of CO is
very low and decreases from 1.4% at 623 K to 0.8% at 698 K.
Assuming that CO2 and CO are formed via water-gas shift
reaction, the selectivity of CO is calculated thermodynamically to
be 1.6% at 623 K and 17.0% at 698 K. It is likely that water-gas
shift reaction does not take place under this reaction condition and
CO2 is formed through direct reaction between water and
methanol or methanol-derived species adsorbed on SnO2 surface.
The catalyst was inspected by XRD after the steam reforming at
698 K. The XRD pattern did not change from that observed with
the fresh catalyst and showed the presence of only SnO2 phase.
However, diffraction lines assigned to metallic Sn phase were
observed with the catalyst exposed to the mixture of nitrogen and
methanol at the same temperature (698 K). The facts show that the
reduction of SnO2 into Sn metal is inhibited by the presence of
water.
2CH3OH ! CO2 þ CH4 þ 2H2
ð1Þ
SnO2 catalyst used in this study was obtained from Kanto
Chemical Co., and was calcined at 723 K for 2 h in nitrogen prior
to the reaction. The catalytic reaction was carried out with a fixed
bed flow type reactor at an atmospheric pressure. The typical
reaction conditions were; temperature, 623 K; N2/CH3OH/H2O
feed ratio, 2/1/1; total flow rate, 37.5 cm3 minꢁ1; and catalyst
amount, 0.97 g. The products in the effluent stream were analyzed
by on-line gas chromatograph equipped with a thermal con-
ductivity detector and a flame ionization detector. The catalysts
before or after the reaction were characterized by X-ray powder
diffraction (XRD).
Figure 1 shows the effect of reaction temperature on the
steam reforming over SnO2 catalyst. The reaction was conducted
3hours at every temperature and activity change was not observed
appreciably during the reaction. A substantial amount of
hydrogen was detected in the effluent gas but was not analyzed
for every run quantitatively. The main product was CO2,
suggesting that the steam reforming of methanol proceeded
predominantly. The selectivity was as high as 80% independently
Figure 2 shows the effect of contact time (W/F) on the steam
reforming at 698 K. W and F represent the amount of catalyst and
the total flow rate, respectively. The data were collected by
varying the amount of catalyst and by fixing the total flow rate of
the feed. As seen in the figure, the conversion of methanol and
CO2 selectivity increased with increasing W/F, while HCHO
selectivity decreased. The increase in CO2 selectivity corre-
sponds well to the decrease in HCHO selectivity, suggesting that
HCHO is an intermediate of this particular steam reforming.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
1079
Figure 3. Effect of partial pressure of water on the steam
reforming of methanol over SnO2. Reaction temperature, 623 K;
catalyst, 0.49 g; total flow rate, 35.5–55.9 cm3 minꢁ1; N2/
CH3OH/H2O, 4/1/0–2.7/1/1.3.
Figure 2. Effect of contact time on the steam reforming of
methanol over SnO2. Reaction temperature, 623 K; catalyst,
0.123–1.94 g; total flow rate, 37.5 cm3 minꢁ1; N2/CH3OH/H2O,
2/1/1.
proceed through the steam reforming of HCHO formed by the
dehydrogenation of methanol and not to be accompanied by the
water-gas shift reaction.
The steam reforming was carried out at 623 K by varying the
partial pressure of water. The mixture of methanol and water was
fed at various molar ratios and the partial pressure of methanol
was maintained at 0.02 MPa by adjusting the flow rate of nitrogen.
Accordingly, the total flow rate changed from 35.5 to
55.9 cm3 minꢁ1, but the values of methanol conversion fell in
the range of 6.2–9.4%. Figure 3 shows the rate of methanol
consumption and the selectivity of each product as a function of
partial pressure of water. In the absence of water, the selectivity of
CH4 was approximately equal to that of CO2, as reported
previously.11 The rate of methanol consumption was not affected
largely by the presence of water vapor regardless of its partial
pressure. The fact suggests that the rates of methanol conversion
(eq 1) and steam reforming are controlled by the same elementary
step, probably the dehydrogenation of methanol to HCHO. The
selectivity of HCHO and CH4 decreased with increasing the
partial pressure of water while the selectivity of CO2 increased. At
higher partial pressure of water, HCHO formed seems to react
with water preferentially to form CO2 and H2 through an
intermediate like formic acid or formate species, and with
methanol to some extent to form HCOOCH3 which readily
decomposes to CO2 and CH4. The selectivity of CO was very low
at the whole range of water partial pressure.
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Although the reaction mechanism is still under investigation,
this steam reforming of methanol over SnO2 is considered to